A transconductance amplifier is the amplifier that supplies an output current proportional to an input voltage, and generally has steady gain (or transconductance). In other words, the output current varies in proportion to the input voltage as varied over a predetermined range of operating input voltages, or equivalently, the output current is linear with respect to the input voltage.
The approach of using a pair of MOS transistors having a common source, as shown in FIG. 1, for example, is known as a conventional transconductance amplifier having good linearity between the input voltage and the output current in the predetermined range of operating input voltages. (See Non-patent Document 1.) Amplifiers 106 and 107, and MOS transistors 113 and 114 cascode-connected connections to MOS transistors 111 and 112, respectively are used so as to keep the drain voltages of the MOS transistors 111 and 112 constant to input variations at all times. Also, the transistor sizes (each of which is the ratio between a channel width and a channel length) of the MOS transistors and the tuning voltage values Vctrl and common voltages Vcm thereof are controlled by a voltage generator circuit 100 and a fixed voltage generator 119. This makes the MOS transistors 111 and 112 that form a differential pair operate in a triode region and the MOS transistors 113 and 114 operate in a saturation region. Moreover, input voltages Vip and Vin are controlled by a differential-pair input voltage generator circuit 120. The differential-pair input voltage generator circuit 120 is supplied with an input voltage Vinput and the common voltage Vcm, outputs the voltage Vip to a gate terminal of the MOS transistor 111, and outputs the voltage Vin to a gate terminal of the MOS transistor 112.
FIG. 2 shows the relationship between the input voltage and transconductance Gm obtained by differentiating the output current with respect to the input voltage, which is observed in the conventional transconductance amplifier. From FIG. 2, it can be seen that the transconductance Gm is held constant in the vicinity of zero (Vip−Vin=0) and the output current is proportional to the input voltage. The tuning voltage Vctrl may be also controlled for adjustment of the transconductance Gm, while maintaining the linearity between the input voltage and the output current. In FIG. 2, there is shown the transconductance Gm observed at the tuning voltage Vctrl as changed from a middle level to a low level and to a high level.
However, the conventional transconductance amplifier, such as shown in FIG. 1, has the problem that an increase in the tuning voltage Vctrl for the adjustment of the transconductance leads to deterioration in the linearity of the transconductance amplifier between the input voltage and the output current, as shown in FIG. 2. In other words, the range of the transconductance Gm being held constant varies depending on the magnitude of the tuning voltage Vctrl. Thus, it is necessary that the range of operating input voltages be narrowed or that the amount of change in the tuning voltage Vctrl be reduced to narrow the range of adjustment of the transconductance Gm, in order to permit the adjustment of the transconductance Gm while maintaining the linearity between the input voltage and the output current throughout the entire range of operating input voltages.
Non-patent Document 1: Behzad Razavi, translation supervised by Tadahiro Kuroda, “Design of Analog CMOS Integrated Circuits,” Maruzen Co., Ltd., Jul. 30, 2005, p. 559